14,747 research outputs found
Non-equilibrium properties of the S=1/2 Heisenberg model in a time-dependent magnetic field
The time-dependent behavior of the Heisenberg model in contact with a phonon
heat bath and in an external time-dependent magnetic field is studied by means
of a path integral approach. The action of the phonon heat bath is taken into
account up to the second order in the coupling to the heath bath. It is shown
that there is a minimal value of the magnetic field below which the average
magnetization of the system does not relax to equilibrium when the external
magnetic field is flipped. This result is in qualitative agreement with the
mean field results obtained within -theory.Comment: To be published in Physica
Another analytic view about quantifying social forces
Montroll had considered a Verhulst evolution approach for introducing a
notion he called "social force", to describe a jump in some economic output
when a new technology or product outcompetes a previous one. In fact,
Montroll's adaptation of Verhulst equation is more like an economic field
description than a "social force". The empirical Verhulst logistic function and
the Gompertz double exponential law are used here in order to present an
alternative view, within a similar mechanistic physics framework. As an
example, a "social force" modifying the rate in the number of temples
constructed by a religious movement, the Antoinist community, between 1910 and
1940 in Belgium is found and quantified. Practically, two temple inauguration
regimes are seen to exist over different time spans, separated by a gap
attributed to a specific "constraint", a taxation system, but allowing for a
different, smooth, evolution rather than a jump. The impulse force duration is
also emphasized as being better taken into account within the Gompertz
framework. Moreover, a "social force" can be as here, attributed to a change in
the limited need/capacity of some population, coupled to some external field,
in either Verhulst or Gompertz equation, rather than resulting from already
existing but competing goods as imagined by Montroll.Comment: 4 figures, 29 refs., 15 pages; prepared for Advances in Complex
System
Correlation amplitude and entanglement entropy in random spin chains
Using strong-disorder renormalization group, numerical exact diagonalization,
and quantum Monte Carlo methods, we revisit the random antiferromagnetic XXZ
spin-1/2 chain focusing on the long-length and ground-state behavior of the
average time-independent spin-spin correlation function C(l)=\upsilon
l^{-\eta}. In addition to the well-known universal (disorder-independent)
power-law exponent \eta=2, we find interesting universal features displayed by
the prefactor \upsilon=\upsilon_o/3, if l is odd, and \upsilon=\upsilon_e/3,
otherwise. Although \upsilon_o and \upsilon_e are nonuniversal (disorder
dependent) and distinct in magnitude, the combination \upsilon_o + \upsilon_e =
-1/4 is universal if C is computed along the symmetric (longitudinal) axis. The
origin of the nonuniversalities of the prefactors is discussed in the
renormalization-group framework where a solvable toy model is considered.
Moreover, we relate the average correlation function with the average
entanglement entropy, whose amplitude has been recently shown to be universal.
The nonuniversalities of the prefactors are shown to contribute only to surface
terms of the entropy. Finally, we discuss the experimental relevance of our
results by computing the structure factor whose scaling properties,
interestingly, depend on the correlation prefactors.Comment: v1: 16 pages, 15 figures; v2: 17 pages, improved discussions and
statistics, references added, published versio
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
The Landless Voices Database: A Trajectory from Cultural Studies to Pedagogical Impact
This article initially addresses the conception of the web-enabled database Landless Voices (VIEIRA, 2003) as a contribution to Cultural Studies and, crucially, to the understanding of the relational workings of Brazil’s complex and regionally diverse culture of landlessness, and to the validation of the cultural self-expression of the sem-terra/Sem Terra. Secondly, it analyzes speculative data obtained from prospective primary teachers being trained at the Federal University of Paraná, Brazil, on the contribution of the database to the pedagogical context. The main findings are that the three predominant types of impact, using Meagher’s terminology (2013, p. 5) are conceptual (new knowledge), cultural (revising misconceptions) and instrumental (future development of pedagogic practices). It then moves to empirical research on social impact, understood as the contribution of academic research to non-academic users, more specifically to its presumably main beneficiaries – the Sem Terra learners themselves. The findings of these first exploratory workshops with learners in four rural schools in settlements in the states of Paraná and São Paulo, respectively in October and November 2013, are that the database broadens their educational resources and empowers a historically marginalized social segment. This article confirms, however, that impact is not a punctual activity (MEAGHER, 2013) and concludes on the need for continuous interaction with learners for the pedagogic impact of academic research to be generated[1].[1] This is part of Landless Voices Impact Enhancement Project, developed by Professor Else R. P. Vieira (Principal investigator) and co-researchers: Dr. Sônia Schwendler (Federal University of Paraná, UFPR) and Professor Bernardo Mançano Fernandes (São Paulo based UNESCO’s Chair in Territorial Development and Education for the Countryside; MA in Territorial Development in Latin American and Caribbean of the São Paulo State University – UNESP, and PhD Programme in Geography, Presidente Prudente Campus), both honorary researchers at Queen Mary’s Department of Iberian and Latin American Studies. This research on impact is financed by the School of Languages, Linguistics and Film (Queen Mary University of London) with the support of UFPR School of Education and of UNESP Centre for Agrarian Reform Studies, Research and Projects − NERA)
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